For the purposes of the following description, user authentication typically is understood as the act of proving to a computer-based system that a user is who she or he claims to be. User authentication is often based on providing user credentials in terms of three factors:                something you know (e.g., a password),        something you have (e.g., an ATM card), and        something you are (e.g., a fingerprint).        
One or more of these factors are typically verified to authenticate a user. For example, a computer user is required to authenticate him- or herself, when, e.g., logging into his computer account, retrieving e-mail from servers, accessing certain files, databases, networks, web sites, etc. In banking applications, a bank account holder is required to provide his automatic teller machine (ATM) card as a first factor and enter a personal identification number (PIN) as a second factor in order to access the automated teller machine to conduct a banking transaction.
With the increased use of wearable user devices and in particular wearable smart and/or computing devices, it is feasible to use such wearable device in a user authentication system, e.g., for determining whether to authenticate a user for a certain application or whether such authentication should be revoked, depending on the particular circumstances. A corresponding authentication system is disclosed in the above-mentioned utility patent application according to the publication US 2009/0249478 A1.
Since an authentication procedure may be conducted for granting access to restricted information or resources, a security concern may arise in case a wearable device of a user that is used for authentication purposes is stolen, e.g. by a malicious party.
Accordingly, an object exists to provide an improved wearable user device for use in a user authentication system, which enhances the security of a user authentication.